It is proposed to develop a quantitative assay for carcinogen-induced transformation in mouse epidermal cell cultures, using the failure to terminally differentiate in response to high calcium levels as an in vitro end-point. Cell strains with desired properties will be derived from mouse epidermis and cultured in 0.02 mM Ca++ medium. Responses to carcinogens and non-carcinogens will be tested by quantitating numbers of cells failing to differentiate in 1.2 mM Ca++ medium. Optimal conditions will be evaluated and dose-response curves constructed. In attempts to define the carcinogenic or mutagenic nature of the induced transformation, the response to tumor promotors, and a quantitative assessement of the concurrent induction of mutations at the HGPRT and ouabain loci will be made. To further define the assay, carcinogen-transformed cells will be characterized by various histochemical and ultrastructural markers, and for their growth requirements in vitro. Tumorigenicity will be assessed in conventional animals and in "nude" mice at various times with and without exposure to a tumor promoter. Having defined optimum conditions and the end-point to the assay attempts will be made to inhibit transformation using corticosteroids and retinoids shown to have in vivo tumor inhibitory activity. These studies are designed to develop a model for the study of epithelial carcinogenesis. In humans, cancer at this site accounts for the vast majority of neoplasms and is known to be susceptible to environmental carcinogenesis.